Sarah Pidgeon's colourists spent "over 20 hours" to achieve Carolyn Bessette Kennedy's iconic blonde shade.
The task proved challenging because the late fashion publicist - who Sarah, 29, plays in FX's Love Story: John F. Kennedy Jr. and Carolyn Bessette - had a hair colour and style evolution throughout the 1990s
Kari Hill, celebrity colourist and Schwarzkopf Professional ambassador, explained to People: "In the early ’90s, Carolyn's blonde was higher contrast - darker underneath with bold, warm highlights layered over the surface.
"It had presence and confidence, with visible dimension that felt intentional rather than blended away.
"By the late ’90s, her blonde became brighter and more buttery, but she still maintained that deeper dimension at the base. It never looked flat or overly toned - there was always depth beneath the brightness.
"Taking Sarah from very dark hair to Carolyn’s blonde was a full transformation - it took over 20 hours in the first session alone."
Sarah's hair health was the number one priority for Kari.
Kari said: "Not all bleach is created equal, and lifting that much pigment can be dangerous without the right technology.
"I used Schwarzkopf Professional BLONDME 9+ Premium Lightener because its Anti-Metal Bond Protection Technology allowed me to lift the colour safely without her hair breaking off.
"I then toned the hair with Schwarzkopf Professional Igora Vibrance, whose formula is fortified with Vitamins B3 and B5 to shield the hair’s surface during the colouring process and to leave the hair beautifully blended, silky and soft."
Another challenge was replicating Carolyn's hair texture.
Barry Lee Moe, Love Story set stylist, said: "Her effortless, lived-in waves still stand out today, and it was important to bring that to life in order to fully encapsulate her transformation from the early days at Calvin Klein into the Kennedy years."
And Sarah slept in braids each night before filming Love Story to get the correct texture.
Barry added: "I generously applied Unite 7-Second Leave-In Conditioner from mid-lengths to ends, and sectioned her hair into four quadrants, loosely braiding it off until the next morning.
"This technique created a beautiful, soft wave that I further defined with a waving iron and random, tapered vertical spirals throughout, so that nothing felt too uniform."
Barry then used Velcro rollers to achieve the volume in Sarah's roots before finishing her Bessette look with "Unite Texturiza spray to create a voluminous, lived-in vibe and Unite Blonda hair oil to lock in moisture, reduce frizz and add shine".
And he hailed the actress' transformation into Carolyn - who died aged 33 in a plane crash alongside her husband, JFK Jr, 38, and her sister, Lauren Bessette, 34, off Martha's Vineyard, Massachusetts, in July 1999 - as an "incredible journey".
Barry added: "From the moment she stepped out of the salon, her entire energy changed, and everyone noticed.
"I think she was truly born to be blonde, and thanks to Kari Hill and Alex Pardoe, we were able to make it a reality. I’ll never forget our first day back on set with all the elements in place.
"It was magical to see how much the hair elevated her already incredible performance. Everything just aligned, and Sarah transformed."
